Method for producing silane

ABSTRACT

The invention relates to a method for producing silane (SiH 4 ) by a) reacting metallurgical silicon with silicon tetrachloride (SiCl 4 ) and hydrogen (H 2 ), to form a crude gas stream containing trichlorosilane (SiHCl 3 ) and silicon tetrachloride (SiCl 4 ), b) removing impurities from the resulting crude gas stream by washing with condensed chlorosilanes, c) condensing and subsequently, separating the purified crude gas stream by distillation, d) returning the partial stream consisting essentially of SiCl 4  to the reaction of metallurgical silicon with SiCl 4  and H 2 , e) disproportionating the partial stream containing SiHCl 3 , to form SiCl 4  and SiH 4  and f) returning the SiH 4  formed by disproportionation to the reaction of metallurgical silicon with SiCl 4  and H 2 , the crude gas stream containing trichlorosilane and silicon tetrachloride being liberated from solids as far as possible by gas filtration before being washed with the condensed chlorosilanes. The washing process with the condensed chlorosilanes is carried out at a pressure of 25 to 40 bar and at a temperature of at least 150° C. in a single-stage distillation column and is carried out in such a way that 0.1 to 3 wt. % of the crude gas stream containing trichlorosilane and silicon tetrachloride is recovered in the form of a condensed liquid phase consisting essentially of SiCl 4 , this liquid phase then being removed from the SiCl 4  circuit and expanded to a pressure of 1 bar outside said SiCl 4  circuit and cooled to a temperature of 10 to 40° C., whereby dissolved impurities separate out and are then removed by filtration.

[0001] The present invention relates to a method for producing silane(SiH₄) by reacting metallurgical silicon with silicon tetrachloride(SiCl₄), hydrogen (H₂) and hydrogen chloride (HCl), removing impuritiesfrom the resulting crude gas stream containing trichlorosilane (SiHCl₃)and disproportionating the said SiHCl₃ to form SiCl₄ and silane.

[0002] Silane can be used for the manufacture of high-purity silicon asrequired for the manufacture of semi-conductors and solar cells.According to “Silicon for the Chemical Industry IV, Geiranger, Norway,Jun. 3-5, 1998, Ed.: H. A. Øye, H. M. Rong, L Nygaard, G. Schüissler, J.Kr. Tuset, p. 93-112” silane required for the manufacture of high-puritysilicon is produced by two different methods:

[0003] Reacting silicon tetrafluoride (SiF₄) with sodium aluminiumhydride (NaAlH₄) to form SiH₄ and sodium aluminium fluoride (NaAlF₄),purifying the produced SiH₄, separation of high-purity silicon onsilicon seed crystal in a fluidized bed and removal of H₂ from theformed high-purity silicon granules. Large amounts of NaAl₄ occur inthis process which must be utilized or marketed accordingly.

[0004] Reaction of metallurgical silicon with SiCl₄ and H₂ in afluidized bed to form SiHCl₃, catalysed two-stage disproportionation ofsaid SiHCl₃ to form SiCl₄ and SiH₄, returning the SiCl₄ formed bydisproportionation to the reaction of metallurgical silicon with SiCl₄and H₂, thermal decomposition of the formed SiH₄ on silicon rods to formhigh-purity silicon and returning the H₂ formed by decomposition to thereaction of metallurgical silicon with SiCl₄ and H₂.

[0005] The latter method is characterized in that the inevitableproduction of large amounts of by-products is avoided because the SiCl₄occurring in the process is used for the manufacture of SiHCl₃ byreacting said SiCl₄ with metallurgical silicon and hydrogen.

[0006] Embodiments of said method are specified in “Studies in OrganicChemistry 49, Catalyzed Direct Reactions of Silicon, Elsevier, 1993, p.450 to 457”, DE 3 311 650 C2 und CA-A-1 162 028. According to the abovedocumentation, the manufacture of silane according to the said methodcomprises the following steps:

[0007] 1. Reacting of metallurgical silicon with SiCl₄ and H₂ attemperatures from 400 to 600° C. and a pressure from 20.7 to 41.4 bar ina fluidized-bed reactor.

[0008] 2. Removing impurities, such as not reacted fine silicon, metalchlorides, polysilane, siloxane and, if necessary, catalyst, from theresulting reaction mixture containing chlorosilane and hydrogenous acidby washing the hot crude gas stream with condensed chlorosilanes.

[0009] 3. Removing the resulting chlorosilane suspension containingsolids.

[0010] 4. Condensing the purified reaction mixture.

[0011] 5. Returning the hydrogen formed in Step 4 in Step 1.

[0012] 6. Separating the purified reaction mixture by distillation toform SiCl₄ and SiHCl₃.

[0013] 7. Returning the SiCl₄ in Step 1.

[0014] 8. Two-stage catalysed disproportionation of the SiHCl₃ obtainedin Step 6 to form SiH₄ and SiCl₄.

[0015] 9. Returning the SiCl₄ in Step 1.

[0016] 10. Removing impurities by distillation from the SiH₄ obtained inStep 8.

[0017] A disadvantage of the specified method is that the removal ofimpurities from the hot gas stream resulting from the reaction in thefluidized bed by washing with condensed chlorosilanes (Step 2) istechnically very expensive due to the presence of fine solid gascomponents. There is further the risk that the employed apparatus may bechoked by solids which makes a continuous operation difficult.

[0018] The chlorosilane suspension resulting from Step 2 containingsilicon metal and metal chloride is removed in accordance with DE 3 709577 A1 by a specific separation of chlorosilanes and solids bydistillation whereby a high percentage of chlorosilanes can be recoveredand is returned to the circuit. The remaining distillation bottomproduct containing solids and chlorosilane cannot be utilized and mustbe disposed of in a way as it is specified, for example, in U.S. Pat.No. 4,690,810. This procedure impairs the economic efficiency of themethod. Another disadvantage is that together with the recoveredchlorosilane undesired impurities are returned to the process operatingstate silane production which can result in an undesired concentrationof such impurities affecting the process.

[0019] So characteristic for the method is a returning stream of silicontetrachloride. Combining all relevant equations, silane is produced fromsilicon and hydrogen by this method. Silicon tetrachloride ispermanently circulating during the reaction and does not leave the saidcircuit.

Si+2 H₂+3 SiCl₄→4SiHCl₃  (1)

4 SiHCl₃→3 SiCl₄+SiH₄  (2)

Si+2 H₂→SiH₄  (3)

[0020] Because of the incomplete reaction of silicon, hydrogen andsilicon tetrachloride the first equation should be defined more exact asfollows:

Si+(2+x)H₂+(3+y)SiCl₄→4 SiHCl₃ +×H₂ +y SiCl₄  (1a)

[0021] This does not change the total result of equation (3), but itbecomes visible that the not reacted silicon tetrachloride increases thecircuit stream of silicon tetrachloride.

[0022] The object of the present invention was to provide a method forthe manufacture of silane that is free of the above specifieddisadvantages and allows to produce silane at low costs.

[0023] The present invention relates to a method for producing silane(SiH₄) by

[0024] a) reacting metallurgical silicon with silicon tetrachloride(SiCl₄) and hydrogen (H₂) to form a crude gas stream containingtrichlorosilane (SiHCl₃) and silicon tetrachloride (SiCl₄),

[0025] b) removing impurities from the resulting crude gas stream bywashing with condensed chlorosilanes to produce a purified crude gasstream containing trichlorosilane and silicon tetrachloride and ahomogeneous liquid phase consisting essentially of SiCl₄, this liquidphase then being removed from the circuit,

[0026] c) condensing and subsequently separating the purified crude gasstream by distillation to form a partial stream consisting essentiallyof SiCl₄ and a partial stream consisting essentially of SiHCl₃,

[0027] d) returning the partial stream consisting essentially of SiCl₄to the reaction of metallurgical silicon with SiCl₄ and H₂,

[0028] e) disproportionating the partial stream containingtrichlorosilane to form SiCl₄ and SiH₄, and

[0029] f) returning the SiCl₄ formed by disproportionation to thereaction of metallurgical silicon with SiCl₄ and H₂, characterized inthat the crude gas stream containing trichlorosilane and silicontetrachloride being liberated from solids as far as possible by gasfiltration before being washed with the condensed chlorosilanes, thewashing process with the condensed chlorosilanes is carried out at apressure of 25 to 40 bar and at a temperature of at least 150° C. in amulti-stage distillation column and is carried out in such a way that0.1 to 3 weight percent of the crude gas stream containingtrichlorosilane and silicon tetrachloride is recovered in the form of acondensed liquid phase consisting essentially of SiCl₄, condensed liquidphase consisting essentially of SiCl₄ then being removed from the SiCl₄circuit and expanded to a pressure of 1 bar outside said SiCl₄ circuitand cooled to a temperature of 10 to 40° C., whereby dissolvedimpurities separate out and are then removed by filtration.

[0030] Preferably washing is carried out in such a way that 0.5 to 1.5weight percent of the crude gas stream containing trichlorosilane andsilicon tetrachloride is recovered as in the form of a condensed liquidphase consisting essentially of SiCl₄.

[0031] Metallurgical silicon in this meaning refers to siliconcontaining up to approx. 3 weight percent iron, 0.75 weight percentaluminium, 0.5 weight percent calcium and other impurities as canusually be found in silicon obtained by carbothermal reduction ofsilicon-di-oxide.

[0032] Preferably the reaction of metallurgical silicon with SiCl₄ andH₂ (Step a)) is carried out at temperatures from 500 to 800° C. and apressure from 25 to 40 bar.

[0033] Suitable apparatuses for gas filtration are, for example,cyclones or hot-gas filters. In an advantageous embodiment of the methodaccording to the invention gas filtration is carried out in severalcyclones which are connected in series or in multi-cyclones. Such filterapparatuses are specified for example in Ullmann's Encyclopedia ofIndustrial Chemistry, Volume B 2, Unit Operation 1, 5^(th) completerevised Edition, VCH-Verlagsgesellschaft, Weinheim p. 13-4 to 13-8.Alternatively also hot-gas filters with sintered metal or ceramiccandles or combinations of cyclones and hot-gas filters can be used.Using the above mentioned filter apparatuses ensures that the solids areseparated from the crude gas stream as far as possible enabling anunobstructed subsequent washing with the condensed chlorosilanes. In themethod according to the invention impurities that are still contained inthe crude gas stream after gas filtration, such as metal chlorides,non-metal chlorides, siloxanes and polysilanes, are separated in thecondensed liquid phase consisting essentially of SiCl₄ and can easily beremoved together with it from the process of silane production.

[0034] Another advantage is that by this method a solid containingsilicon metal is obtained which can be used in metallurgical processes,such as e.g. the manufacture of iron alloys, due to its high siliconcontents. To this end the solid containing silicon metal and metalchloride can be reacted for example with alkaline compounds, such assoda lye, Na₂CO₃, NaHCO₃ and CaO and water, filtered and washed withwater to remove chloride and dried if necessary.

[0035] Preferably the liquid phase consisting essentially of SiCl₄resulting from washing with condensed chlorosilanes and the subsequentpressure reduction and cooling is liberated from impurities separatedout by means of plate pressure filters. It is preferred to use sinteredmetals, particularly preferred sintered wire-cloth, as filter elements.Such filter elements are commercially available under the trade namesPoroplate® and Fuji-Plate®. Alternatively decanters can also be used toremove the impurities separating out.

[0036] The resulting filtrate is excellently suitable as raw materialfor the manufacture of pyrogenic silicic acid and is thereforepreferably used for this purpose. Any further reprocessing, e.g. bydistillation, is not required. The solid resulting from filtration canbe inerted in the known way with alkaline compounds, such as soda lye,Na₂CO₃, NaHCO₃ and CaO and used after inerting as raw material in themanufacture of cement.

[0037] In a particularly preferred embodiment of the method according tothe invention the need of chloride equivalents caused by the dischargeof filtrate essentially consisting of SiCl₄ is compensated by adding0.05 to 10 weight percent hydrogen chloride (HCl), based on the amountof SiCl₄ introduced, as an additional reactand in reacting metallurgicalsilicon with SiCl₄ and H₂. Preferably an amount of 0.5 to 3 weightpercent HCl is used.

[0038] Using an amount of 0.5 to 10 weight percent HCl, based on theamount of SiCl₄ introduced, as additional reactand causes an unexpectedacceleration of the reaction finally resulting in a very high yield ofSiHCl₃, that means high reaction rates near the thermodynamicequilibrium of the SiCl₄ employed, and at the same time high totalyields, i.e. a largely complete utilization of the metallurgical siliconemployed.

[0039] Hydrogen chloride is preferably used in an anhydrous form ashydrogen chloride gas.

[0040] Hydrogen chloride, for example, cannot be introduced separatelyin the reactor where the reaction to form trichlorosilane will becarried out. It is also possible, however, to introduce hydrogenchloride in the reactor together with gaseous and/or vaporisablestarting materials hydrogen and/or silicon tetrachloride.

[0041] The preferred embodiment of the method according to the inventioncomprising the addition of hydrogen chloride when reacting metallurgicalsilicon with SiCl₄ and H₂ is characterized mainly in that anacceleration of the reaction is caused by using the inventive amounts ofHCl as additional reactand thus achieving a higher utilization ratio ofthe metallurgical silicon used and improving the economic efficiency ofthe method considerably.

[0042] So the addition of the preferably to be added amount of hydrogenchloride causes a faster activation of silicon. When fresh silicon isreacted with hydrogen and silicon tetrachloride or such reaction iscontinued after an interruption of the process, an induction periodoccurs lasting approx. 100 minutes, for example, in case of a reactiontemperature of 600° C. and a H₂: SiCl₄ mol ratio of 2:1. Such inductionperiod is reduced to 45 minutes by addition of 2 weight percent hydrogenchloride based on silicon tetrachloride and apart from this the sameconditions. Since in a reaction carried out in a continuously operatedfluidized-bed reactor a considerable part of the fluidized bed alwayscontains freshly introduced silicon the faster activation of suchsilicon has an accelerating effect on the process as a whole.

[0043] Furthermore, adding hydrogen chloride causes the reaction gassesto attack all over the complete silicon area. When adding solidcatalysts to the silicon to be reacted the reaction with thehydrogen/silicon tetrachloride gas occurs immediately at the edge of thecatalyst corns resulting in crater-like cavities. Upon further progressof the reaction into the depth, the silicon surfaces that werepreviously covered with catalyst particles are undermined and theparticles disengage from the silicon corn and are subsequently carriedaway as small particles from the fluidized bed. Thus neither the saidsilicon particles which were carried away, nor the adherent catalystsare available for the desired reaction. The consequences are a worsetotal yield and a decreasing reaction velocity of the silicon particledepleted of catalyst. The same occurs, in principle, in the uncatalysedreaction of silicon with hydrogen/silicon tetrachloride without additionof hydrogen chloride. In this case the reaction proceeds in crater-likecavities along the band formed by the impurities separating out. Suchband contain the impurities contained in silicon or introduced by theraw materials and the production process, essentially iron, aluminium,calcium, titanium, causing also an acceleration of the reaction. Incontrast, the reaction of silicon with hydrogen/silicon tetrachloride inthe presence of hydrogen chloride occurs all over the surface of thesilicon corns forming a high number of crater-like cavities on thesurface of such silicon corns. Since the complete surface of the siliconcorns provides essentially more reaction area than the sections occupiedby catalyst particles and/or the bands on the edges of the cornscontaining the collected impurities, the silicon area participating inthe reaction is much bigger thus causing an acceleration of the reactionvelocity depending on the area.

[0044] The preferred embodiment of the method according to the inventioncomprising the addition of hydrogen chloride when reacting metallurgicalsilicon with SiCl₄ and H₂ is also characterized in that the amount ofsilicon carried out undesiredly is reduced. Reacting on large surfacesof the silicon corns in the presence of hydrogen chloride prevents theundermining of the spots containing catalyst and the corn-burstingtrenching reaction along the bands containing impurities preventing theblasting off of small silicon particles and their being carried out ofthe fluidized bed by the reaction gasses. This increases the yield oftrichlorosilane based on the amount of silicon tetrachloride used aswell as based on the silicon used.

[0045] In addition to this, adding hydrogen chloride results in aconstant reaction velocity in the course of the decomposition ofsilicon. Contrary to the catalysed reaction without addition of hydrogenchloride the reaction velocity of the silicon/hydrogen/silicontetrachloride reaction is not reduced considerably in the presence ofhydrogen chloride. This finding is surprising, since the silicon cornsare getting smaller in the course of the reaction which should cause areduction of surface of a given amount of silicon, and thesurface-related residence time of the reaction gasses decreases as well.

[0046] The selection of the reactor for the reaction according to theinvention is not critical, provided that under the reaction conditionsthe reactor shows adequate stability and permits the contact of thestarting materials. The process can be carried out, for example, in afixed bed reactor, a rotary tubular kiln or a fluidized-bed reactor. Itis preferred to carry out the reaction in a fluidized-bed reactor.

[0047] The material of the reactor must resist the reaction conditionsmentioned for SiHCl₃ synthesis. The requirements on the resistance ofthe construction materials of the reactor apply also for any precedingand secondary parts of the plant, such as for example cyclones or heatexchangers. These requirements are fulfilled, for example, by nickelbase alloys.

[0048] Further acceleration of the reaction of metallurgical siliconwith SiCl₄, H₂ and HCl, if applicable, can be achieved by the use ofcatalysts. Particularly suitable catalysts are copper, iron, copper oriron compounds or any mixtures thereof.

[0049] Surprisingly it was found that the catalysts unfold particularlyhigh efficiency when the metallurgical silicon is provided in a milledform and was mixed intensively with the catalysts prior to the reaction.

[0050] It is therefore preferred in the method according to theinvention to carry out the reaction to form trichlorosilane (Step a)) inthe presence of catalyst, and to mix the metallurgical siliconintensively with the catalysts prior to the reaction.

[0051] Preferably the silicon is provided in fine form, particularlypreferred with an average grain diameter of 10 to 1000 μm, moreparticularly preferred of 100 to 600 μm. The average grain diameter iscalculated as the arithmetical mean of the values determined in a sieveanalysis of the silicon.

[0052] Preferably, the mixing of catalyst and silicon is carried out inapparatuses ensuring a very intensive mixing. Particularly suitable forthis purpose are mixers provided with rotary mixing tools. Such mixersare specified for example in “Ullmann's Encyclopedia of IndustrialChemistry, Volume B2, Unit Operations I, p.27-1 to 27-16, VCHVerlagsgesellschaft, Weinheim”. Particularly preferred is the use ofplough blade mixers.

[0053] During intensive mixing the catalyst can be milled further whichresults in a very good distribution of the catalyst during mixing and avery good adherence of the catalyst on the silicon surface. Thereforealso catalysts can be used which are not available in a very fine formor cannot be milled to the required fineness.

[0054] In the case of insufficient mixing a large portion of catalyst isdirectly carried out of the fluidized bed together with the gaseousreactands and/or products due to poor adherence of catalyst to siliconparticles and is therefore not available for the reaction any more. Thiscauses an increased demand for catalyst impairing the economicefficiency of the method. This is avoided by intensive mixing of siliconand catalyst.

[0055] Preferably the time for mixing silicon and catalyst is 1 to 60minutes. As a rule, longer mixing times are not required. Particularlypreferred are mixing times from 5 to 20 minutes.

[0056] Intensive mixing of catalyst and silicon can be carried out forexample in an inert atmosphere or in the presence of hydrogen or othergasses with a time-reducing effect, e.g. carbon monoxide. This preventsformation of an oxidic layer on the individual silicon particles. Suchlayer prevents direct contact between catalyst and silicon which wouldresult in a poorer catalysing of the reaction with silicontetrachloride, hydrogen and, if necessary, hydrogen chloride totrichlorosilane.

[0057] An inert atmosphere can be achieved, for example, by adding aninert gas during mixing. Suitable inert gasses are, for example,nitrogen and/or argon.

[0058] Particularly preferred is the mixing of silicon and catalyst inthe presence of hydrogen.

[0059] On principle, all catalysts known for reacting silicon withsilicon tetrachloride, hydrogen and, if necessary, hydrogen can be usedas catalyst.

[0060] Particularly suitable catalysts are copper and iron catalysts.Examples for this are copper oxide catalysts (e.g. Cuprokat®,manufacturer: Norddeutsche Affinerie), copper chloride (CuCl CuCl₂),copper metal, iron oxides (e.g. Fe₂O₃, Fe₃O₄), ferrous chlorides (FeCl₂,FeCl₃) and their mixtures.

[0061] Preferred catalysts are copper oxide catalysts and iron oxidecatalysts.

[0062] Particularly when using copper oxide catalysts and iron oxidecatalysts it has proved advantageous to mix the silicon at a temperaturefrom 100 to 400° C., preferably from 130 to 250° C. This removes anymoisture residue adherent to catalysts which would otherwise have anegative impact on the reaction of silicon with SiCl₄, H₂ and HCl, ifapplicable. Mixing in the presence of reducing gasses, preferablyhydrogen, furthermore reduces oxidic components of the catalystspreventing a yield reduction caused by oxygen or oxides when reactingmetallurgical silicon with SiCl₄ and H₂. Furthermore a better adherenceof catalyst to the silicon surface is achieved by this method avoidinglargely any catalyst loss in the fluidized bed.

[0063] It is also possible to use mixtures of copper catalysts and/oriron catalysts with further catalytically active components. Suchcatalytically active components are, for example, metal halogenides,such as e.g. chlorides, bromides or iodides of aluminium, vanadium orantimony.

[0064] Preferably the amount of catalyst used, calculated as metal, is0.5 to 10 weight percent, particularly preferred 1 to 5 weight percent,based on the silicon employed.

[0065] The mol ratio of hydrogen to silicon tetrachloride when reactingmetallurgical silicon with SiCl₄ und H₂ can be for example 0.25:1 to4:1. A mol ratio of 0.6:1 to 2:1 is preferred.

[0066] The partial stream resulting from the separation of the purifiedcrude gas stream containing trichlorosilane and silicon tetrachloride bydistillation consisting essentially of SiHCl₃ is disproportionatedpreferably in a column at a pressure from 1 to 10 bar, wherein thecolumn provides at least two reactive/destillative reaction zones.

[0067] Such disproportionation is carried out on catalytically activesolids, preferably in catalyst beds each one consisting of a layer ofbulk material of said catalytically active solids which can be streamedthrough by the products of the disproportionation. Instead of a layer ofbulk material also packed catalyst bodies can be provided in thereaction zone.

[0068] Suitable catalytically active solids are known and specified, forexample, in DE 2 507 864 A1. Such suitable solids, for example, aresolids carrying amino groups or alkyleneamino groups on a structure ofpolystyrene, crosslinked with divinylbenzole. Suitable amino groups oralkylenamino groups are for example: dimethylamino, diethylamino,ethylmethylamino, di-n-propylamino, di-iso-propylamino,di-2-chlorethylamino, di-2chlorpropylamino groups and the respectivehydrochlorides, or the trialkylammonium groups formed from them bymethylation, ethylation, propylation, butylation, hydroxyethylation orbenzylation with chloride as counterion. Of course, in the case ofquaternary ammonia salts or protonized ammonia salts also catalyticallyactive solids with other anions, e.g. hydroxide, sulphate, bisulphate,bicarbonate etc. can be introduced into the method according to theinvention, a transformation into the chloride form, however, isinevitable under the reaction conditions in the course of time, thisapplies also to organic hydroxy groups. Therefore, ammonia saltscontaining chloride as counterion are preferred.

[0069] Also those solids are suitable as catalytically active solidswhich consist of a structure of polyacrylic acid, particularly apolyacrylamide structure, that has bound, for example,trialkylbenzylammonium via an alkyl group.

[0070] Another suitable group of catalytically active solids are, forexample, solids carrying sulphonate groups on a structure ofpolystyrene, cross-linked with divinylbenzole, which are opposed bytertiary or quaternary ammonium groups as cations.

[0071] As a rule, macroporous or mesoporous ion exchangers are moresuitable than gel resins.

[0072] Preferably the method according to the invention is integratedinto a general method for producing hyper-pure silicon.

[0073] Particularly preferred, the method according to the invention isintegrated into a multistage general method for producing hyper-puresilicon, as specified for example in “Economics of Polysilicon Process,Osaka Titanium Co., DOE/JPL 1012122 (1985), 57-78” and comprising thefollowing steps:

[0074] a) Production of trichlorosilane;

[0075] b) Disproportionation of trichlorosilane to yield silane;

[0076] c) Purifying silane to obtain high-purity silane, and

[0077] d) Thermal decomposition of silane in a fluidized-bed reactor anddepositing of hyper-pure silicon on the silicon particles which form thefluidized bed.

[0078] The particular advantages of adding hydrogen chloride whenreacting metallurgical silicon with SiCl₄ and H₂, as it is carried outin Step a) of a preferred embodiment of the method according to theinvention, is being explained in more detail in the following examples.The examples shall not be understood, however, as a restriction to theinventive idea insofar.

EXAMPLE 1a

[0079] 400 g silicon (99.3 weight percent silicon, average diameter ofparticles Dp=250-315 μm) were provided in a fluidized-bed reactor withan internal diameter (I.D.) of 0.05 m and were reacted with hydrogen andsilicon tetrachloride at a temperature T=600° C. and a total pressure ofP_(tot)=1.1 bar. The mol ratio H₂/SiCl₄ was 2 in the presence of 20volume percent N₂. The reaction was carried out under addition of 2weight percent HCl, based on the amount of silicon tetrachloride. Thetime for achieving 95% of stationary yield of trichlorosilane wasT_(95%)=45 min.

EXAMPLE 1b (COMPARATIVE EXAMPLE)

[0080] The reaction according to Example 1a was repeated, however,without adding HCl. The time for achieving 95% of stationary yield oftrichlorosilane was T_(95%)=100 min.

EXAMPLE 2a

[0081] 400 g silicon (99.3 weight percent silicon, average diameter ofparticles Dp=250-315 μm) were provided in a fluidized-bed reactor withan internal diameter (I.D.) of 0.05 m and were reacted with hydrogen andsilicon tetrachloride at a temperature T=600° C. and a total pressure ofP_(tot)=1.1 bar. The mol ratio H₂/SiCl₄ was 2 in the presence of 20volume percent N₂. The reaction was carried out several days. An amountof 1.5±1-0.5 weight percent based on the amount of silicon tetrachloridewas added continuously. After 24.2% of silicon were reacted the reactionmass was examined by means of surface electron microscopy. It shows thata reaction of silicon had occurred all over the surface of the siliconparticles.

EXAMPLE 2b (COMPARATIVE EXAMPLE)

[0082] The reaction according to Example 2a was repeated, this time,however, without adding hydrogen chloride. After 23.4% of silicon werereacted the reaction mass was examined by means of surface electronmicroscopy. It can be recognized that the silicon reacted only on singlepoints and edges.

EXAMPLE 2c (COMPARATIVE EXAMPLE)

[0083] Similar to Example 2a, 400 g of silicon (99.3 weight percentsilicon, average diameter of particles Dp=160-195 μm) were provided in afluidized-bed reactor with an internal diameter (I.D.) of 0.05 m andwere reacted with hydrogen and silicon tetrachloride at a temperatureT=600° C. and a total pressure of P_(tot)=1.1 bar. The mol ratioH₂/SiCl₄ was 2 in the presence of 20 volume percent N₂. The reaction wascarried out several days. But no HCl was added, the reaction was carriedout instead in the presence of 1 weight percent Cu in form of Cumetal/Cu₂O/CuO as catalyst. After 31.4% of silicon were reacted thereaction mass was examined by means of surface electron microscopy. Itshows that a slight reaction of silicon occurred, but also underminingof the surface and formation of big cavities can be observed.

EXAMPLE 3a

[0084] Example 2 a was repeated, but this time the reaction was notinterrupted after 24.2% of silicon had reacted. The amount of materialcarried away was determined by means of the amounts separated in acyclone. The amount of material carried away decreased continuouslyaccording to the degree of reacted silicon and was below 0.5 weightpercent based on the amount of reacted silicon already when a degree of15% of reacted silicon was reached.

EXAMPLE 3b (COMPARATIVE EXPERIMENT)

[0085] Example 2 b was repeated, but this time the reaction was notinterrupted after 23.4% of silicon had reacted. The amount of materialcarried away was determined by means of the amounts separated in acyclone. The amount of material carried away increased up to a degree ofreacted silicon of approx. 15% and then decreased. At a degree ofreacted silicon of approx. 15% the amount of material carried away wasabove 1.0 weight percent based on the amount of reacted silicon.

EXAMPLE 3c (COMPARATIVE EXPERIMENT)

[0086] Example 2 c was repeated, but this time the reaction was notinterrupted after 31.4% of silicon had reacted. The amount of materialcarried away was determined by means of the amounts separated in acyclone. The amount of material carried away increased continuouslyaccording to the degree of reacted silicon and was above 1.0 weightpercent based on the amount of reacted silicon already when a degree of15% of reacted silicon was reached. At a degree of reacted silicon ofapprox. 45% amounts of material carried away of above 7.0 weight percentbased on the amount of reacted silicon were observed.

[0087] A comparison of the amount carried away in a reaction accordingto Examples 3a, 3b and 3c is depicted in FIG. 1 specifying the amountcarried away (A) based on the amount of reacted silicon in weightpercent in comparison with the degree of reacted silicon (X) in %. Thedesignations 3a, 3b, 3c of the graphs correspond to the numbering of theexamples.

EXAMPLE 4

[0088]400 g silicon (99.3 weight percent silicon, average diameter ofparticles (Dp) in uncatalysed reaction and under addition of HCl:Dp=250-315 μm; Cu-catalysed reaction: Dp=160-195 μm) were provided in afluidized-bed reactor with an internal diameter (I.D.) of 0.05 m and thehydrogen-chloride reaction was carried out at a temperature T=600° C.and a total pressure of P_(tot)=1.1 bar for several days. The mol ratioH₂/SiCl₄ was 2 in the presence of 20 volume percent N₂. Three forms ofreaction were carried out: a) not catalysed without adding HCl(comparison), b) Cu-catalysed without adding HCl (1% Cu as Cumetal/Cu₂O/CuO) (comparison) and c) without Cu catalyst and withadditional adding of 1.5±0.5 weight percent of HCl. In the differentforms of the reaction the yield of the target product trichlorosilanewas determined. It shows that the yield in a reaction according to theinvention including the addition of HCl is not decreasing asconsiderably during the reaction as in the case of the uncatalysed or Cucatalysed reactions without addition of HCl.

1. Method for producing silane (SiH₄) by a) reacting metallurgicalsilicon with silicon tetrachloride (SiCl₄) and hydrogen (H₂) to form acrude gas stream containing trichlorosilane (SiHCl₃) and silicontetrachloride (SiCl₄), b) removing impurities from the resulting crudegas stream by washing with condensed chlorosilanes to produce a purifiedcrude gas stream containing trichlorosilane and silicon tetrachlorideand a homogeneous liquid phase consisting essentially of SiCl₄, thisliquid phase then being removed from the circuit, c) condensing andsubsequently separating the purified crude gas stream by distillation toform a partial stream consisting essentially of SiCl₄ and a partialstream consisting essentially of SiHCl₃, d) returning the partial streamconsisting essentially of SiCl₄ to the reaction of metallurgical siliconwith SiCl₄ and H₂, e) disproportionating the partial stream containingSiHCl₃ to form SiCl₄ and SiH₄, and f) returning the SiCl₄ formed in thereaction to the reaction of metallurgical silicon with SiCl₄ and H₂,characterized in that the crude gas stream containing trichlorosilaneand silicon tetrachloride is liberated from solids as far as possible bygas filtration before being washed with the condensed chlorosilanes, thewashing process with the condensed chlorosilanes is carried out at apressure of 25 to 40 bar and at a temperature of at least 150° C. in amulti-stage distillation column and is carried out in such a way that0.1 to 3 weight percent of the crude gas stream containingtrichlorosilane and silicon tetrachloride is recovered in the form of acondensed liquid phase consisting essentially of SiCl₄, such condensedliquid phase consisting essentially of SiCl₄ is removed from the SiCl₄circuit and expanded to a pressure of 1 bar outside said SiCl₄ circuitand cooled to a temperature of 10 to 40° C., whereby dissolvedimpurities separate out, and such impurities separating out are removedby filtration.
 2. A method according to claim 1, characterized in thatthe gas filtration is carried out in several cyclones which areconnected in series or in one multi-cyclone.
 3. A method according to atleast one of claims 1 to 2, characterized in that the impuritiesseparating out are removed from the liquid phase consisting essentiallyof SiCl₄ by filtration by means of plate pressure filters provided withsintered wire-cloth as filter element.
 4. A method according to claim 3,characterized in that the filtrate consisting essentially of SiCl₄ isused as raw material for the manufacture of pyrogenic silicic acid.
 5. Amethod according to at least one of claims 1 to 4, characterized in that0.05 to 10 weight percent hydrogen chloride (HCl), based on the amountof SiCl₄, is used as an additional reactant in reacting metallurgicalsilicon with SiCl₄ and H₂.
 6. A method according to at least one ofclaims 1 to 5, characterized in that 0.5 to 3 weight percent HCl, basedon the amount of SiCl₄ introduced, is used as an additional reactant inreacting metallurgical silicon with SiCl₄ and H₂.
 7. A method accordingto at least one of claims 1 to 6, characterized in that the hydrogenchloride to be added is used in an anhydrous form as hydrogen chloridegas.
 8. A method according to at least one of claims 1 to 7,characterized in that the reaction of metallurgical silicon with SiCl₄and H₂ is carried out in the presence of a catalyst.
 9. A methodaccording to claim 8, characterized in that the catalysts used arecopper, iron, copper compounds, iron compounds or any mixtures thereof.10. A method according to at least one of claims 1 to 9, characterizedin that the reaction of metallurgical silicon with SiCl₄ and H₂ iscarried out at temperatures from 500 to 800° C. and a pressure from 25to 40 bar.